Chris Finill knows the London Marathon route better than almost anyone. Since the annual race began in 1981, he's one of 11 people who have run every single one.

On Sunday, he'll complete the course, which starts in the south east of the capital and snakes west until it reaches Buckingham Palace, for the 39th time. Last year he was tripped after just five of the 42.2 kilometers, and broke his arm in four places. He still finished in three hours and fifty-four minutes. But despite his experience the course, which should seem straightforward, can still prove problematic.

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There are a few big challenges for runners taking on the route. All of these can mess with runners to slow them down, stopping them from setting personal bests. Two of these big problems – the hills and the amount of corners – aren't going to change before race day. But the other, the weather, is much less predictable.

When the marathon starts on April 28, there will be two races taking place. One for the elite, professional runners and wheelchair athletes with big prize money and world records at stake. The other is the more familiar mass participation event where more than 40,000 people will pound the tarmac of central London. The runners will start in timed waves based on their ability but all set off from three points around Blackheath, in south east London.

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From there, runners all converge on the same piece of tarmac heading towards Woolwich. And there can be a lot of congestion when the groups come together. "The first major hazard for most people is running very close to other people," Finill says.

"The danger is you get runners who want to come through because they think they're running too slowly and other ones who, although they could run more quickly, want to stick to a predefined pace," he adds. This is exactly what caused his fall in 2018. "You can get a bit of ducking, weaving and diving."

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But added to the surge at the start of the race, there's also an energy-sapping hill. And it’s the downhill that takes its toll. The starting point is about 40 meters above sea level and there's an instant climb to 54m. From here it is downhill at the 4km mark (around three miles) with a drop from of 30m. The downhill can cause runners to start their races running too quickly, meaning their legs tire more towards the end of the marathon distance.

How to watch and live stream the London Marathon 2019

What time does the race start?

The London Marathon is made-up of a number of races. The elite wheelchair race starts at 09:05 BST, this is followed by the Para Athletics Marathon Championships at 09:10, the elite women at 09:25 and finally the elite men and mass start from 10:10.

Is there a London Marathon live stream?

In the UK, there'll be BBC coverage of the races. This will be on BBC One and BBC Two from 08:30. It'll also be possible to stream the coverage through the iPlayer and BBC website. The BBC's coverage is set to run until 14:30, with extended highlights also appearing elsewhere after the race. In the US, NBC will be live streaming the race on its website and app.

How to track runners

Every runner taking part in the London Marathon wears an electronic chip that acts as a tracker. This is activated each time a person passes over a timing station. Friends and family can track runners through the London Marathon app by entering their race number or name.

"Mile three is the one for people to watch out for," says Gareth Mills, the UK manager for sports social network Strava. By crunching data from around 14,000 people who logged their London Marathon performance last year, Strava found that the third mile had the fastest averages times for both male and female runners. Mills says: "Everyone's training kicks in, everyone has worked really hard to get to the start line and people run too hard in mile three."

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After the big drop, the hills and dips facing runners are fairly small. With the marathon largely being based around the Thames it isn't that much above sea level. Finill describes the 42.2km route as being "undulating". The biggest rises come as runners pass the halfway point at Tower Bridge and then a slight incline going towards the concrete dystopia of Canary Wharf at the 27km (17 mile) marker, before dragging runners along Embankment next to the Thames.

What can be problematic is the number of turns. "If you look at the overall thing it is very twisty and turning," says Andy Milroy, a statistician at the Road Runners Club and long-distance running historian. "The turns will actually slow people down and create bottlenecks." The first tight turn is in Greenwich as the route loops around the Cutty Sark. The next array of corners is at Canary Wharf where the course weaves between the island's tower blocks. According to Strava data, in 2018 the slowest average mile was number 22, on the way out of Canary Wharf and going past the Limehouse tube station.

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Despite the bends and slight hills, London is still a fast marathon. It just depends where you are in the race. Elite runners have set some the fastest times in history running in the city. The men's course record, 2:03:05, was set by world record holder Eliud Kipchoge in 2016. It's the third-fastest marathon of all time. On nine other occasions a man has finished in under two hours and five minutes (two more of those belong to Kipchoge).

Kipchoge's world record of 2:01:39 was set in Berlin last year as he took a huge one minute and 20 seconds off the existing record of Dennis Kimetto's 2014 time. Berlin is generally a flatter course than London, with its lowest point being 37m above sea level and the highest at 53m. London, by contrast, ranges from a couple of meters above sea level to the mid 50s.

Of the 50 fastest men's marathons of all time London appears in the list four times. To appear in the rankings courses have to meet international criteria for not only distance but also the changes in elevation and locations of start/finish points. Berlin takes eight of the top ten fastest times, with London and Dubai appearing once each. (Boston, the world's oldest annual marathon, has seen two runners produce times that would appear in the top five of all time but the course doesn't qualify as a record eligible route).

Women running the London Marathon fare a little better. Nine of the fastest 50 races ever have been run in the capital. Paula Radcliffe's 2:15:25, set in London in April 2003, still tops the charts as the world record and is still unlikely to be beaten for some time. In 2017 Kenya's Mary Keitany finished the 42.2km in 2:17:01, breaking Radcliffe's women's only world record. (The British runner's fastest ever time was set using male pacers).

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The second, sixth and seventh fastest marathons were also run in London with Berlin first appearing as the eight fastest of all time. In total, on 11 occasions women have run under 2:20 in London. Three of these fastest times all belong to Radcliffe, with only two other runners being able to go sub two hours 18 minutes on the course.

"It is a fast course, it is fairly flat and it does lend itself to people at the front of the course running fast times," Strava's Mills says. "But because of the amount of runners as well we find its not necessarily a personal best course for runners further down the pack." Instead, he says Manchester is one of the best courses in the UK for runners hoping to get a new PB.

University of Dublin computer science professor Barry Smyth has anlaysed 1.2 million marathon finish times from around the world. The result? Out of nine big city marathons, London has one of the slowest mean average finishing times, for all competitors, for both men and women. According to the data, the Boston Marathon is the quickest. But there's a catch.

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Pretty much the only way runners can compete in Boston is by posting a fast qualifying time – under 3:05 for men aged 18 - 34 and 3:35 for women in the same age bracket. London and other leading courses including Berlin, Chicago, Tokyo and New York, have a lot more places open to a public ballot, charity entries and people in fancy dress.

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But wherever a runner comes in the London marathon pack, there’s one thing they can’t control. The weather. The 2018 race was the hottest on record, hitting peaks of 24.1C. This will slow down even the fastest runners. Last year’s Boston Marathon saw surprise winners when heavy rain, freezing temperatures and sleet saw the predicted leaders drop out early. The weather for the 2019 London Marathon? The scorching temperatures aren’t forecast to return. At the time of writing it is set to be 14C and cloudy. A temperature that’s almost ideal for running.

London has one more weather trick up its sleeve to that could scupper runners: the wind. At a warmup half marathon on some of the London course in May, a person dressed as the Queen Elizabeth Tower struggled to make it through Canary Wharf when gusts of wind swirled between the office blocks. “You'll be going through a built-up environment and the wind will be channel in some areas,” Milroy says.

It’s not just around the built-up areas though. On the final miles to the finish line there’s also a decent chance of winds slowing runners down. Finill says: “Generally the wind is against runners in the last five or six miles because it tends to blow in from the west. So you can be a little bit exposed.”